1. Field of the Invention
The present invention relates to a color image forming apparatus including tandem type image forming units that form images of different colors, respectively, and are disposed along a conveying direction of a conveyor belt, and including transfer units that transfer the images of different colors formed by the image forming units to the conveyor belt or a transfer sheet conveyed on the convey belt while superimposing each of the images on one another.
2. Discussion of the Background
A so-called tandem-type color image forming apparatus has been widely used. In the tandem-type color image forming apparatus, a plurality of image forming units that respectively form images of different colors are disposed along a conveying direction of a conveyor belt that conveys transfer sheets or images. The images of different colors formed by the image forming units are sequentially transferred to a transfer sheet or the conveyor belt while being superimposed on one another. In this tandem-type color image forming apparatus, when transferring images of different colors to a transfer sheet or a conveyor belt, the images may tend to have displacements relative to each other, resulting in a blurred color image. Therefore, to avoid position displacements among color images, a color image forming position is adjusted by performing position displacement detections every time a predetermined number of prints are output. For example, published Japanese Patent application No. 2002-207338 describes a position displacement detection technique.
In a position displacement detection operation, position displacement detecting test marks are formed on a conveyor belt, and are read by optical sensors. Further, a position displacement detection operation is performed based on a level of a detection signal of each of the optical sensors.
Because image quality is adversely affected not only by position displacements among images but also by density variations among the images, a density of the color image is adjusted by performing image density detections. In an image density detection operation, test marks of all colors are formed on a conveyor belt, and are read by optical sensors which also read test marks formed for a position displacement detection operation. Further, the density of a toner image is adjusted based on a level of a detection signal of each of the optical sensors.
The above-described position displacement detection operation and the image density detection operation are performed by forming different types of test marks. When both the position displacement detection operation and the image density detection operation need to be performed, two types of test marks are formed on the conveyor belt and read by the optical sensors sequentially. It takes time to complete adjustment operations for a color image forming position and for the density of a toner image. Consequently, a user has to wait for some time until a color image forming apparatus becomes ready for image formation.
To reduce the above-described user's waiting time until the completion of adjustment operations for a color image forming position and for the density of a toner image, a background image forming apparatus includes two types of sensors; one for reading test marks formed in the position displacement detection operation, and the other for reading test marks formed in the image density detection operation. However, the cost of the apparatus increases due to the increase in the number of optical sensors.
Therefore, it is desirable to provide a color image forming apparatus and a method of controlling the color image forming apparatus that can efficiently perform a position displacement detection operation and an image density detection operation without increasing the user's waiting time and without incurring an increase in costs.